Signal strength meter drive circuit

ABSTRACT

A signal strength meter drive circuit for a radio receiver or tuner comprises a bypass resistor R 3  connected in shunt with the output resistor R 1  of a mirror circuit for an IF amplifier output summing transistor Q 10  through a transistor Q 32  whose conduction is controlled by the biasing voltage for the circuit. The mirror circuit current drawn through the bypass resistor regulates the voltage drop across the output resistor, thus compensating for variations in the voltage characteristics of the various circuit elements, such as the Zener voltage, when they are incorporated in a monolithic IC chip.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a signal strength meter drive circuit for a radio receiver or tuner, capable of absorbing variations in the voltage characteristics of diodes, transistors, and the like formed in an Integrated Circuit (IC).

2. Description of the Prior Art

Conventionally, a diode level shifting circuit has been employed in the signal strength meter drive circuit of a radio receiver or tuner. When such a circuit is fabricated as an IC, however, variations in the voltage characteristics of the contained elements caused by temperature fluctuations and differences in the diffusing conditions are not compensated for, whereby when the amount of the level shift is large the meter may not indicate any output despite the fact that there is a sufficient antenna input, while when the amount of the level shift is small an offset in the meter output may occur.

The circuit diagram of a conventional signal strength meter is shown in FIG. 1, wherein an intermediate frequency amplifier IF₁ includes differential amplifier transistors Q₂₄ and Q₂₅, whose emitters are connected to a constant current source transistor Q₂₃. The output of amplifier IF₁ is half-wave rectified by a capacitor C₁ and transistors Q₁, Q₂ and Q₃. Similarly, the output of amplifier IF₂ is half-wave rectified by a capacitor C₂ and transistors Q₄, Q₅ and Q₆, and the output of amplifier IF₃ is half-wave rectified by a capacitor C₃ and transistors Q₇, Q₈ and Q₉. The collectors of Q₃, Q₆ and Q₉ are connected to the emitter of transistor Q₁₀, whose collector current thus represents the sum of the half-wave rectified outputs of amplifiers IF₁ to IF₃. Transistors Q₁₁ to Q₁₃ provide constant current biasing for Q₁ - Q₉. Transistors Q.sub. 14, Q₁₅ and Q₁₆ constitute a current mirror circuit, whereby the current flowing through a resistor R₁ is the same as the collector current of Q₁₀. The voltage across R₁ is thus proportional to the collector current of Q₁₀, and this voltage is level shifted by a circuit comprising transistors Q₁₇, Q₁₈, diode D₁ and a resistor R₂ to provide a signal strength meter output at terminal OUT.

The base biasing of Q₁₀ is stabilized by a circuit comprising transistors Q₁₉ to Q₂₂ and diodes D₂ to D₅, and the base of Q₂₀ is connected to the bases of Q₂₃, Q₂₆ and Q₂₉ to provide constant current biasing therefor.

In the above-described circuit, assuming that the voltage characteristic across resistor R₁ with respect to the antenna input level is designated by curve f in FIG. 3, such characteristic will vary between curves e and g due to variations in the characteristics of transistors Q₁₉ to Q₂₂ and/or diodes D₂ to D₅ in the biasing circuit. When the Zener diode D₃ is formed in a monolithic IC, such variation is particularly pronounced and cannot be prevented. Consequently, the output voltage at terminal OUT varies as indicated in FIG. 4 between curves e' and g'. For example, when the voltage across resistor R₁ has a characteristic as shown by curve e in FIG. 3, the voltage characteristic at terminal OUT is as shown by curve e' in FIG. 4. Thus, even if the antenna input level is zero, the meter is activated and indicates an output, which is termed "offset". Further, then the voltage characteristic of resistor R₁ is as shown by curve g in FIG. 3 due to variation error, the circuit output voltage assumes the characteristic shown by curve g' in FIG. 4, whereby the meter reads zero over a certain range when there is actually a small antenna input signal present. Even if the accuracies or characteristics of transistors Q₁₉ to Q₂₂, diodes D₂ to D₅, or capacitors C₁ to C₃ are enhanced so that the width between curves e' and g' in FIG. 4 is narrowed, variations in the characteristics of the active elements Q₁₇, Q₁₈ and D₁ would still tend to widen the gap between curves e' and g'. Consequently, every circuit element would have to be highly accurate, and from a practical point of view this is extremely difficult and expensive.

SUMMARY OF THE INVENTION

Briefly, and in accordance with the present invention, a signal strength meter drive circuit for a radio receiver or tuner comprises a bypass resistor connected in shunt with the output resistor of a mirror circuit for an IF amplifier output summing transistor through a transistor whose conduction is controlled by the biasing voltage for the circuit. The mirror circuit current drawn through the bypass resistor regulates the voltage drop across the output resistor, thus compensating for variations in the voltage characteristics of the various circuit elements, such as the Zener voltage, when they are incorporated in a monolithic IC chip.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a schematic diagram of a signal strength meter drive circuit according to the prior art,

FIG. 2 shows a schematic diagram of such a drive circuit according to the present invention, and

FIGS. 3, 4 and 5 show antenna input level vs. D.C. voltage characteristics at various points in the circuits of FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 2, the elements identical to those in FIG. 1 are designated by the same reference characters, and their description will not be repeated. A constant current circuit comprising transistor Q₃₂ and resistor R₃ is connected in parallel with resistor R₁, and operates as a shunt circuit with respect thereto. The base of transistor Q₃₂ is connected to the base of transistor Q₂₀, whereby the base biasing of transistor Q₃₂ is common to that of IF amplifiers IF₁ to IF₃.

If the base potential of transistor Q₁₉ increases due to a variation in the Zener diode D₃, the base potential of Q₂₀ also increases, which in turn increases the base biasing of the IF amplifiers IF₁ to IF₃. Thus, the IF outputs are increased and their summed output current in transistor Q₁₀ is increased, whereby the current through and voltage across resistor R₁ are correspondingly increased. By the provision of the shunt circuit comprising transistor Q₃₂ and resistor R₃, however, some of the current which would normally flow through resistor R₁ is shunted through resistor R₃, which thus subtracts from or reduces the current flowing through R₁. The amount of such reduction is of course proportional to the conduction level of Q₃₂, which in turn is proportional to any variation in the voltage level or characteristic of D₃. Consequently, variations in the Zener diode D₃ are compensated for or absorbed. Further, since the level shifting circuit in FIG. 2 comprises only one transistor Q₁₇, output fluctuations due to variations produced by the level shifting circuit are decreased.

According to the present invention, the width of the variations in the voltage characteristic across resistor R₁ substantially narrowed, particularly when the antenna input level is low, as can be seen from the separation between curves e" and g" in FIG. 5. Accordingly, the fidelity of the signal strength meter indication is enhanced, thus compensating for drive circuit drift when the circuit elements are formed in an IC chip, or the like. 

What is claimed is:
 1. In a signal strength meter drive circuit for a radio receiver or tuner including a plurality of intermediate frequency amplifiers, means for rectifying the outputs of said amplifiers, amplifier means for summing said rectified outputs, said amplifier means including a load resistor, an output circuit for shifting the level of the voltage across said load resistor for driving said meter, and circuit means for supplying biasing voltages to the above circuit components, the improvement characterized by:shunt circuit means connected in parallel with said load resistor and including variable conduction means controlled by said biasing circuit means, whereby said shunt circuit means draws off a proportion of said load resistor current to thereby compensate for variations in the voltage characteristics of the various circuit components.
 2. A drive circuit as defined in claim 1, wherein said shunt circuit means comprises a resistor and a transistor connected in series.
 3. A drive circuit as defined in claim 1, wherein said biasing circuit means comprises at least one Zener diode, and at least some of said circuit components are incorporated in a monolithic integrated circuit.
 4. A drive circuit as defined in claim 2, wherein said biasing circuit means comprises at least one Zener diode, and at least some of said circuit components are incorporated in a monolithic integrated circuit.
 5. A drive circuit as defined in claim 1, wherein said summing amplifier means comprises a summing amplifier and a mirror circuit therefor, and said load resistor is connected in the output of said mirror circuit.
 6. A drive circuit as defined in claim 2, wherein said summing amplifier means comprises a summing amplifier and a mirror circuit therefor, and said load resistor is connected in the output of said mirror circuit.
 7. A drive circuit as defined in claim 3, wherein said summing amplifier means comprises a summing amplifier and a mirror circuit therefor, and said load resistor is connected in the output of said mirror circuit.
 8. A drive circuit as defined in claim 4, wherein said summing amplifier means comprises a summing amplifier and a mirror circuit therefor, and said load resistor is connected in the output of said mirror circuit. 